1. Field of Technology
The present invention relates to a wavelength separator used in a non-emissive display device such as a liquid crystal television and back-lighted sign boards, and high-luminance planar illumination device and liquid display device using this wavelength separator and a laser light source.
2. Description of the Background Art
A liquid crystal display device adopts a method for displaying an image by controlling a transmission amount of light irradiated from behind utilizing an electrooptic effect by the orientation of liquid crystal molecules. A general liquid crystal display device includes a planar illumination device called a backlight unit formed by fluorescent display tubes.
In recent years, screens of such liquid display devices have become larger and display devices for televisions of 50 inches and larger have been put to practical use. However, since power consumption has also been increased as screens have become larger, technological development for realizing lower power consumption is hoped for and it is strongly demanded to thin display devices in order to maximally reduce indoor spaces taken up by display devices.
In order to deal with lower power consumption and a thinner construction, it has been studied to use light-emitting diodes and lasers with good emission efficiency as light sources, and planar illumination devices using LEDs as light sources and liquid crystal display device using LEDs as light source have been already put to practical use.
For example, Japanese Unexamined Patent Publication No. 2006-185891 discloses a construction in which a diffractive optical element is formed on a side surface of a light guide plate, LED light is incident on this side surface and reflected by an inclined reflecting plate formed on one principal surface of the light guide plate and light with little luminance nonuniformity is emitted via a prism sheet arranged on another principal surface.
In order to realize low power consumption, it has been also studied to improve the transmittance of liquid crystal panels, which has been conventionally about 5%. Polarizing plates and color filters can be cited as key factors to reduce transmittance in liquid crystal panels. Normally, if unpolarized white light is incident on a liquid crystal panel, a transmission amount is reduced to ½ in a polarizing plate and further reduced to ⅓ of the reduced transmission amount in a color filter since any one of red light (R light), green light (G light) and blue light (B light) is selected in each filter region and passes through the filter region.
A construction for inserting a polarization reflection sheet between a liquid crystal panel and a backlight unit has been put to practical use as a method for improving transmittance in a polarizing plate. Specifically, the polarization reflection sheet transmits necessary polarized light components while reflecting unnecessary polarized light components. The light reflected by the polarization reflection sheet has the polarization thereof rotated to be converted into effective polarized light components for recycling.
As a method for improving transmittance in a color filter, a liquid crystal display device for effectively condensing irradiated light separated into R light, G light and B light by a wavelength separating means such a diffraction grating to specified pixels of a liquid crystal panel is proposed, for example, Japanese Unexamined Patent Publications No. 2000-241812, No. H09-113903 and No. H10-253955.
Further, a construction for performing wavelength separation into R light, G light and B light by an interference filter type mirror having wavelength selectivity and formed in a light guide plate and efficiently guiding these R light, G light and B light to specified pixels of a liquid crystal panel is proposed in Japanese Unexamined Patent Publication No. 2006-12722.
Besides, improvements of color filters themselves have been studied and there have been proposed color filters different from conventional absorptive color filters. For example, a color filter constructed such that thickness is partially changed for each region in a film structure of an interference filter to provide a different spectral characteristic for each region has been proposed in Japanese Unexamined Patent Publication No. 2008-170979. Color reproducibility is improved by this color filter.
Further, a color filter using a subwavelength grating is proposed in Japanese Unexamined Patent Publication No. 2007-41555. This subwavelength grating is a diffraction grating having a periodic structure shorter than light wavelength. There has been known a phenomenon in which, if light is incident on such a periodic structure, only light of a certain specific wavelength resonates to be strongly reflected. By using a manufacturing method (nano-imprinting) for pressing a mold with a microscopic convexo-concave pattern of nano scale (nm scale) against a material to be processed, a thin color filter having a large area and formed with a subwavelength grating can be manufactured at a low price and with high throughput.
However, no specific construction for reducing power consumption is disclosed or effect is insufficient in the above conventional constructions.
Although the efficiency of the light source is improved by using the LEDs in Japanese Unexamined Patent Publication No. 2006-185891, no specific construction for improving the transmittance of the liquid crystal panel is disclosed.
Although the construction for performing wavelength separation using diffraction and guiding light to the respective pixels of the liquid crystal panel is disclosed in Japanese Unexamined Patent Publications No. 2000-241812, No. H09-113903 and No. H10-253955, efficiency is not sufficiently improved and a further improvement in efficiency is difficult.
Specifically, in Japanese Unexamined Patent Publications No. 2000-241812 and No. H09-113903, grating pitch needs to be reduced to about several fold of wavelength for separation into R light, G light and B light by diffraction angle differences of about several degrees. If such a construction is employed, it becomes difficult to sufficiently improve diffraction efficiency.
Further, since a volume hologram is used in Japanese Unexamined Patent Publication No. H10-253955, wavelength separation is possible by specified angle differences. However, since an incident angle on the volume hologram is limited, the efficiency of light emerging from the light guide plate is reduced.
Although the construction in which the mirror for performing wavelength separation is formed in the light guide plate is disclosed in Japanese Unexamined Patent Publication No. 2006-12722, the light guide plate of such a structure is difficult to manufacture and costly.
The color filters disclosed in Japanese Unexamined Patent Publications No. 2008-170979 and 2007-41555 are only used in place of the conventional absorptive color filters and do not have largely different light utilization efficiency. Both color filters function as reflective color filters, but no construction for effectively utilizing the reflected light is disclosed.